1. Field of the Invention
The present invention relates to a wide aspect television receiver to reproduce picture images on a wide screen having an aspect ratio of 16:9 from video signals with aspect ratio of 4:3, for example.
2. Description of the Related Art
FIG. 1 is a block diagram of a part of a wide aspect television receiver of the prior art. A luminance signal/chrominance signal separation circuit; (Y/C SEP) 1 separates a composite video signal generated by frequency multiplexing of a luminance signal Y and a chrominance signal C back into the Y signal and the C signal, outputs the Y signal to a video signal process circuit 2 and to a synchronization signal separation circuit (SYNC SEP) 6, and outputs the C signal to a color signal demodulator 3. The video signal process circuit 2 controls the picture quality, contrast and brightness of the Y signal, and outputs the resultant signal to a matrix circuit 4. The color signal demodulator 3 controls the hue and color density of the C signal to generate a color difference signal and outputs the signal to the matrix circuit 4. The matrix circuit 4 generates primary color signals of red, green and blue (RGB) from the luminance signal Y received from the video signal process circuit 2 and the color difference signal received from the color signal demodulator 3. A CRT 5 correctly directs electron beams, which are controlled according to the RGB signals, on the red, green and blue phosphors provided on a fluorescent screen, and thereby reproduces color images.
The SYNC SEP 6 extracts synchronization signals from the Y signal, and outputs a vertical synchronization signal (V-SYNC) among the synchronization signals to a ramp wave generation circuit 7. The ramp wave generation circuit 7 generates a sawtooth wave synchronized with the V-SYNC and outputs the sawtooth wave to a vertical drive circuit (V-DRIVE). The V-DRIVE 8 amplifies the voltage of the sawtooth wave and shapes the waveform, then outputs the wave to a vertical output circuit (V-OUT) 9. The V-OUT 9 amplifies the power of the sawtooth wave and supplies the sawtooth current to a vertical deflection coil 10 which deflects the electron beams vertically on the screen. A vertical deflection linearity correction circuit 13 is provided in a negative feedback loop where the deflection current is negatively fed back to the V-DRIVE 8 in order to correct the linearity of the sawtooth current, and corrects the sawtooth current to be uniformly linear or partially nonlinear, depending on the aspect ratio of the screen where the pictures are to be displayed. Description of circuits of the horizontal deflection system will be omitted here.
The vertical deflection linearity correction circuit 13 is composed of two pairs of Feedback resistances 13a, 13a' resistance value: 13a &gt;13a') and feedback resistances 13b, 13b' (resistance value: 13b&gt;13b'), a switching circuit 13c to switch over the Feedback resistances 13a and 13a' according to the aspect ratio of the screen whereon to reproduce pictures, and a switching circuit 13d to switch over the feedback resistances 13b and 13b' according to the aspect ratio of the screen. The feedback resistances 113a, 13a' are connected in parallel to the positive lead of a coupling capacitor 11 of which a negative lead is grounded via a feedback resistance 12, while the feedback resistances 13b, 13b' are connected in parallel to a lead wire which connects the negative lead of the coupling capacitor 11 and the Feedback resistance 12, via a coupling capacitor 13e. "Direct current component DC+alternate current component AC" is negatively fed back from the positive lead of the coupling capacitor 11, and the alternate component AC is negatively fed back from the negative lead of the coupling capacitor 11, via the vertical deflection linearity correction circuit 13 to the V-DRIVE 8.
By the switching circuits 13c, 13d of the vertical deflection linearity correction circuit 13 switching over the feedback resistances 13a, 13a' and the feedback resistances 13b, 13b', respectively, a raster is formed by a sawtooth current uniformly linear over the entire screen in the vertical direction when reproducing pictures on an ordinary screen, or a raster contracted air the top and bottom thereof is formed while maintaining the aspect ratio (circularity) of the ordinary screen at the center of the screen as shown in FIG. 2, by a linear sawtooth current for the center of the screen and by a nonlinear sawtooth current for the top and bottom of the screen when reproducing pictures on a wide screen.
Now the operation of switching over ordinary screen reproduction and wide screen reproduction by means of the vertical deflection linearity correction circuit 13 will be described below. Because the vertical deflection linearity correction circuit 13 is arranged in the negative feedback loop of a negative feedback amplifier, an increase in the amount of feedback leads to a smaller outpost and a decrease in the amount of feedback leads to a greater output. By making use of this characteristic, the switching circuit 13c switches to the feedback resistance 13a' of a lower resistance value when reproducing pictures on a wide screen. Consequently, an inversely parabolic output as shown in FIG. 3A is obtained because higher parabolic voltage in one vertical scanning period is fed back than that in ordinary screen reproduction. Therefore, such a vertical deflection current flows in the vertical deflection coil 10 as to form a raster expanded at the top and contracted at the bottom.
The switching circuit 13d also switches over to the feedback resistance 13b' of a smaller resistance value. At this time, because more differentiated sawtooth wave obtained in the coupling capacitor 13e is fed back than that in ordinary screen reproduction, the output is smaller only at the start as shown in FIG. 3B so that such a vertical deflection current flows in the vertical deflection coil 10 to form a raster contracted at the top. When these two outputs are synthesized, vertical deflection currents of portions corresponding to the top and bottom of the screen are nonlinear as shown in FIG. 3C, assuming that there is no deflection distortion, a raster having good linearity at the center of the screen and contracted at the top and bottom thereof is formed as shown in FIG. 2.
In ordinary screen reproduction wherein a linear vertical deflection current over the entire screen is supplied, the switching circuit 13c and the switching circuit 13d switch over to the feedback resistance 13a and the feedback resistance 13b which have higher resistance value, respectively. Output of the vertical deflection linearity correction circuit 13 becomes a linear sawtooth wave as indicated by a broken line in FIG. 3C, so that a raster having good linearity over the entire screen is formed.
At the same time, the vertical deflection width is also switched so that the same over scan in the vertical direction as shown in FIG. 4 can be obtained both in ordinary screen reproduction and in wide screen reproduction.
With such a constitution as described above, displaying a picture of aspect ratio 4:3 on a wide screen of aspect ratio 16:9 hardly causes unusual impression of vertical contraction.
However, since the wide aspect television receiver of the prior art displays a picture of aspect ratio 4:3 on a screen of aspect ratio 16:9 by contracting the raster at the top and bottom of the screen while maintaining circularity (aspect ratio) at the center of the CRT screen, only by means of the vertical deflection linearity correction circuit 13 which deals with the linearity of vertical deflection, horizontal scan lines are denser at the top and bottom where the raster is compressed, resulting in higher luminance in these portions of the screen.
Besides, the pincushion distortion on the right and left of the top and bottom of the screen where the raster being contracted is not properly corrected because when the raster is contracted only at the top and bottom after simply broadening the vertical deflection width wider than that in ordinary screen reproduction, contraction at the top and bottom results in a relatively greater influence of the horizontal deflection magnetic field on the top and bottom portions of the screen.
Especially, when using a CRT of high-definition where the pitch of the phosphor stripes on the CRT fluorescent screen or the corresponding pitch is equal all over the screen, the unevenness in luminance at the top and bottom of the screen as described above is conspicuous.